I. Field of the Invention
This invention relates to a process for producing a semiconductor device and, more particularly, to a process for producing a semiconductor device having high quality and self-gettering action.
II. Description of the Prior Art
Large scale integration and high packing density has been recently abruptly advanced for a semiconductor device. A high quality is required for a silicon monocrystalline substrate becoming a material therefor and a process for preventing the contamination thereof in a producing step is also required under such conditions. That is, it is required (1) to equalize the resistivity of the substrate crystal, (2) to employ a gettering process capable of being conducted at relatively low temperature due to the restriction of the depth of a junction, and (3) to remedy for improper memory cell holding time due to diffusion of carrier occurred in the substrate owing to a variety of improper modes of a semiconductor device such as, for example, an alpha particle soft error, an impact ionization and so forth as the respective unit element size is miniaturized.
The present invention will satisfy all these requirements as will be described. As to the above paragraph (1), since an epitaxially grown layer has highly uniform resistivity as compared with a (Czochialski-grown) monocrystal and (floating zone-grown) monocrystal, the equalization of the resistivity of the substrate crystal is conducted by employing the epitaxially grown layer.
Regarding the above paragraph (2), it relates to a gettering of impurities caused in a monocrystal during a device fabrication process (such as, for example, Cu, Te, Au or the like), and a diffusion method with phosphorus, a method of imparting mechanical damage onto the back surface of a wafer, and a method of imparting mechanical damage to the back surface of a wafer due to an ion implantation and so forth have been conducted heretofore. However, all these methods tend to contaminate the wafer and provide weak gettering capacity. Accordingly, an intrinsic gettering method (I.G. method) is taken into consideration as an excellent method. This I.G. method includes the steps of heat treating the wafer to precipitate oxygen atom contained in the crystal, thereby producing micro defects in the crystal, adsorbing the impurities in the surface of a substrate with the micro defects and thus eliminating the defects occurred with the impurities. However, as Katz et al has described on page 1,151 of "High Oxygen Czochralski Crystal Growth Relationship to Epitaxial Stacking Faults" of J. Electrochem. Soc. 125, this method is difficult to obtain uniform distribution of the micro defects made by the precipitation of the oxygen in the wafer. Further, this method has the following drawbacks and disadvantages. This is, (a) the density of the micro defects is very sensitive for the oxygen content and the heat treatment conditions for precipitating the oxygen depend much upon the crystals and cannot be determined according to only one factor. (b) It requires a heat treatment at high temperature for a long time for the formation of a non-defective surface layer. (c) The oxygen precipitation depends upon the temperature hysteresis or the like of growing the crystal, which is difficult to be controlled. (d) When a number of micro defects are produced, it becomes weak against thermal warpage.
It has been proposed to employ an ion implantation process having excellent uniformity and controllability so as to eliminate the above described drawbacks and disadvantages. Rozgonyi et al disclosed a method of gettering by implanting antimony ion on page 1,910 of "The Identification, Annihilation, and Suppression of Nucleation Sites Responsible for Silicon Epitaxial Stacking Faults" of J. Electrochem. Soc. 123. However, this method has such drawback that a part of silicon monocrystal to which antimony ion is implanted becomes N-type semiconductor. When oxygen is used for ion to be implanted, the part of silicon monocrystal to which oxygen ion is implanted does not become N-type nor P-type, and accordingly it can eliminate this drawback. A method of gettering by implanting oxygen ion to a silicon monocrystalline substrate is known and is disclosed in Japanese Patent Disclosure (Kokai) No. 51-32272. Since this method employs the ion implanted silicon monocrystalline substrate as it is without conducting a step of epitaxially growing it, a number of defects occur in the surface layer of semiconductor which should have no defect due to the ion implantation and become harmful for the junction characteristics.